(a) Technical Field
The present disclosure relates, in general, to a method for controlling the amount of air supplied to a fuel cell. More particularly, the present invention relates to a method for controlling the amount of air supplied to a fuel cell, which can prevent flooding and membrane dry-out in a fuel cell stack and, at the same time, ensure optimal performance of the fuel cell stack and a humidifier by supplying an optimal amount of air to the fuel cell stack at each operation condition.
(b) Background
In general, a typical fuel cell system comprises a fuel cell stack for suitably generating electricity by electrochemical reaction, a hydrogen supply system for suitably supplying hydrogen as a fuel to the fuel cell stack, an oxygen (air) supply system for suitably supplying oxygen containing air as an oxidant required for the electrochemical reaction in the fuel cell stack, a thermal management system (TMS) for suitably removing reaction heat from the fuel cell stack to the outside of the fuel cell system, controlling operation temperature of the fuel cell stack, and suitably performing water management function, and a system controller for suitably controlling overall operation of the fuel cell system. Preferably, the fuel cell system generates heat and water as well as electricity.
Preferably, the fuel cell stack consists of a plurality of unit cells, each unit cell preferably including an anode, a cathode and an electrolyte (electrolyte membrane). Preferably, hydrogen is suitably supplied to the anode (“fuel electrode”) and oxygen containing air is suitably supplied to the cathode (“air electrode” or “oxygen electrode”).
In certain preferred embodiments of the present invention, the hydrogen supplied to the anode is dissociated into hydrogen ions (protons, H+) and electrons (e) by a catalyst that is suitably disposed in an electrode/catalyst layer. Preferably, the hydrogen ions are suitably transmitted to the cathode through the electrolyte membrane, which is a cation exchange membrane, and the electrons are suitably transmitted to the cathode through a GDL and a bipolar plate.
Preferably, at the cathode, the hydrogen ions supplied through a polymer electrolyte membrane and the electrons transmitted through the bipolar plate suitably react with the oxygen containing air supplied to the cathode to produce water.
Preferably, migration of the hydrogen ions causes electrons to flow through an external conducting wire, which generates electricity and heat.
In general, the amount of air supplied to the cathode of the fuel cell stack (hereinafter referred to as “stack”) is about two times the stoichiometric ratio (SR). Further, the amount of air supplied to the stack has an effect on power output of the stack, system efficiency, relative humidity of air, and water balance. In particular, when the operation temperature is low such as during fuel cell system start-up or warm-up, flooding (over-condensation) may occur and, when the operation temperature is suitably raised such as during operation at high power, membrane dry-out may occur in the stack.
The reason why the flooding and membrane dry-out occur in the stack is described herein.
Generally, the amount of water suitably generated in the stack is proportional to the amount of current that is suitably generated in the stack. However, the amount of vapor that is contained in the air discharged from the stack differs according to the temperature and pressure of exhaust air.
Accordingly, when the temperature of the stack is suitably low and the pressure is suitably high, the amount of vapor contained in the air is considerably reduced, which means that the amount of water that is not evaporated but condensed in the stack is suitably increased. On the contrary, when the temperature is suitably high and the pressure is suitably low, the amount of vapor contained in the air is rapidly increased (e.g., refer to formula 1).
Accordingly, before the fuel cell is suitably warmed up or before the temperature of the stack reaches a normal level due to low power output, the amount of vapor contained in the air supplied to the stack is suitably reduced such that a substantial amount of water generated is condensed, thus causing flooding in the stack. In particular, the flooding occurring in the stack clogs the air channel or surrounds the catalyst layer, and can cause problems such as deterioration of fuel cell performance.
When the temperature of the stack is suitably high and the pressure is suitably low, the amount of vapor contained in the air is rapidly increased and, since the amount of vapor increased may be suitably greater than the amount of water generated in the stack, the water balance in the stack is broken to cause dry-out in the polymer electrolyte membrane. Accordingly, when the fuel cell is preferably continuously operated in a state where the relative humidity of the electrolyte membrane is suitably low, the polymer electrolyte membrane is suitably dried, and thus the power output and durability of the fuel cell may be suitably reduced.
Accordingly, it is important to properly control the humidity of the polymer electrolyte membrane in the stack and, for this purpose, the air supplied to the cathode of the stack is suitably maintained at an optimal level using a humidifier. However, since the humidifier uses the water generated by the electrochemical reaction of the stack to suitably humidify the air, it is necessary to consider the water balance in the stack.
Accordingly, the water balance may preferably be defined as {(the amount of water generated in the stack)−(the amount of water contained in the exhaust air of the stack (or fuel cell system)}. Accordingly, when the water balance is positive (+), it means that the amount of water in the stack is suitably sufficient, and the water balance is negative (−), it means that the amount of water in the stack is suitably insufficient. It is also possible to maintain the fuel cell in positive water balance when the amount of water generated in the fuel cell is suitably greater than the amount of water contained in the exhaust air.
Preferably, regarding the water balance, it is preferable that the amount of water generated in the fuel cell be greater (+) than the amount of water (vapor) contained in the exhaust water.
Preferably, during start-up, or when the operation temperature is suitably low and the operation pressure is suitably high, the water balance is maintained at a positive level, which means that the amount of vapor contained in the exhaust air is suitably smaller than the amount of water generated in the stack. The flooding (over-condensation) may occur in the stack outlet when the water generated by the electrochemical reaction of the stack is not evaporated but condensed when the absolute humidity is suitably low due to the low operation temperature, and, preferably, it is thus necessary to control the amount of air to suitably prevent the flooding from occurring in the stack outlet.
In particular, when the water generated by the electrochemical reaction is suitably discharged in the form of vapor in the exhaust air, the absolute amount of vapor contained in the air is suitably reduced when the operation temperature is low, which means that the water in the stack may be suitably condensed. Accordingly, it is necessary to prevent the flooding by suitably increasing the amount of air to control the amount of vapor contained in the exhaust air when the operation temperature is suitably low or the temperature pressure is suitably high.
When the operation temperature is suitably high or the operation pressure is suitably low, the amount of vapor contained in the air is rapidly increased to be greater than the amount of water generated in the stack. As a result, the water balance in the stack is suitably broken to cause the dry-out of the polymer electrolyte membrane, and thus it is necessary to suitably control the amount of air.
U.S. Patent Publication No. 20070287041, incorporated by reference in its entirety herein, is directed to a control system for a fuel cell stack that suitably maintains the relative humidity of the cathode inlet air above a predetermined percentage by doing one or more of decreasing stack cooling fluid temperature, increasing cathode pressure, and/or decreasing the cathode stoichiometric ratio when necessary to increase the relative humidity of the cathode exhaust gas that is used by a water vapor transfer device to humidify the cathode inlet air. The control system can also suitably limit the power output of the stack to keep the relative humidity of the cathode inlet air above the predetermined percentage.
However, the control system proposed by the above control system is to maintain the relative humidity of the stack inlet air at a desired level during initial stage and does not consider the operation temperature or pressure. In particular, when the operation temperature of the stack is suitably low, the flooding is most likely to occur in the stack outlet even though the relative humidity of the stack inlet air is suitably maintained at an optimal level. Further, when the operation temperature is high, membrane dry-out is most likely to occur in the stack outlet.
Although there are many methods for variably controlling the amount of air supplied by detecting the temperature and humidity of the stack inlet air, these methods are not directed to controlling the relative humidity of the stack outlet air to a desired level while suitably maintaining the water balance.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.